1. Field of the Invention
This invention relates generally to an impact energy management method and system. More specifically, it relates to a protective structure which is designed to protect an impacted object or body from damage due to impacts and which has properties that are readily customized to provide optimum impact-attenuating responses over a wide range of impact energies.
2. Background Information
A. The Physics of Colliding Objects
An object in motion possesses kinetic energy (KE), which is a function of its mass (m) and velocity (v), described by the equation:KE=½ m v2  (1)
When that object collides with another object, the energy is transferred, imparting a Force (F). The force transmitted is a function of two primary relationships.
First, Force (F) imparted to an object is equal to the object's mass (m) and its resulting acceleration (a), as governed by Newton's Second Law of Motion, Force=mass×acceleration or F=ma. Acceleration (a) measures the object's change in velocity (Δv) over time (t) (change in velocity can be positive or negative, therefore acceleration can represent either a positive or negative quantity), thus Newton's Law can be rewritten as follows:F=m((Δv)/t)  (2)
From this equation, it is apparent that one way to reduce the Force imparted to an object of fixed mass (m) is to prolong the time (t) over which the object changes velocity, thus reducing its acceleration.
Second, Force (F) is a result of the distance (d) over which the object's Energy (E) (in the form of kinetic energy) is transferred, giving the equation:F=E/d  (3)
From this equation, it is apparent that another way to reduce the Force (F) of an impacting object with a given amount of Energy (E) is to prolong the distance (d) over which the object's Energy (E) is transferred.
A third relationship governs the effect of an imparted force. Pressure (P) describes the concentration of Force (F) over the area (A) within which the Force (F) is is imparted and is governed by the equation:P=F/A  (4)
From this equation, it is apparent that the pressure (P) of an impact can be reduced by reducing the Force (F) imparted by the impacting object or by increasing the Area (A) over which that Force (F) is imparted.
Given the above three relationships, it is apparent that the methods to reduce the damage caused by an impacting object are to decrease the level of Force (F) imparted by prolonging the time (t) over which that object accelerates (or decelerates) or the distance (d) over which energy is transferred, or to increase the area (A) over which that Force (F) is spread. An ideal system would employ all three methods to reduce impact damage.
Force is measured in Newtons (1 N=1 kg−m/s2) or pounds (lb): mass is measured in kilograms (kg) or pounds of mass (lb-m): and acceleration is measured in meters per second per second (m/s2) or feet per second per second (ft/s2). A commonly known force is Weight (w) which measures the force of gravity acting on an object. It is equal to the object's mass (m) multiplied by the acceleration due to gravity (g), which is 9.81 m/s2 or 32 ft/s2. When comparing forces that act on objects of the same or similar mass (m), it is common to express them in terms of units of acceleration rather than units of force (recall F=ma). In such cases, acceleration is often expressed as multiples of the acceleration of gravity, or in “g's”. Thus, an object can be said to have experienced an “80-g” force, or a force equal to 80 times the force of gravity. In general, it can be assumed that higher forces are more damaging to an object than lower forces.
In any activity in which two objects are likely to collide, it is common practice to utilize protective structures or materials designed to manage the energy of the collision and to minimize the damage to the impacted object caused by the collision. A common method of testing the efficacy of such protective systems is to impart a known Force (F) to one side of the protective structure or material and to measure the force transmitted through the system to the other side. Often this is accomplished with a “drop test.” In this type of test, an impacting object is dropped (or mechanically accelerated) is from a given height onto a fixed surface, which is adapted to register the force imparted to it by the impacting object. It is typical for the impacted surface to be a steel plate, beneath which is attached a “force ring,” which is capable of registering the forces delivered to the plate, and transmitting a signal representative of the forces to a data capture system, typically a programmed computer. The combination of steel plate and force ring is termed a “force plate.” Thus a useful comparison of protective systems involves placing the energy management system or material onto the force plate, dropping an impacting mass onto the system or material, and registering the forces transmitted through the system or material to the force plate as a function of time.
The greater the height from which an object of fixed mass is dropped, the higher the velocity it will attain before impact, and the more kinetic energy it will possess to transfer to the impacted surface. The force of that impact over time is represented in a Force/Time curve, such as the curve shown in FIG. 1 of the accompanying drawing.
It is important to note that all objects with the same mass and same impact velocity will possess the same amount of energy. The way in which that energy is managed by a protective structure or material will determine the shape of the Force/Time curve. For a given object impacting with a given speed, the area under the Force/Time curve, know as the Impulse (I), will be the same, regardless of the shape of the curve. However, the shape of that curve is a representation of the force profile, which can vary significantly, depending on the energy management system being employed. In general, when managing impacts, the level of peak force attained can be considered to be the most critical indicator of an energy management system's efficacy.
B. Foam as an Impact-Absorbing Material
One of the most common materials used to protect objects from impact forces is foam. Solid foams form an important class of lightweight cellular engineering materials, and are used in many applications where impacts are common, such as in athletic activities (e.g., protective headgear) and automotive applications (e.g., dashboard is coverings). The most general definition of foam is a substance that contains a relatively high volume percentage of small pores, and which is formed by trapping gas bubbles in a liquid or solid. The pores allow foam to deform elastically under impact, and the impact energy is dissipated as the material is compressed. In general, foams decrease impact pressure by spreading forces over a wide area and by prolonging the distance and time over which impacts occur and thus reducing the level of force transmitted.
While foams have been a mainstay in impact protection for decades, they rely solely on material deformation for their energy management capabilities. This presents two major limitations.
First, relying on material properties severely limits the adaptability of the foam. Foams can be customized to respond optimally to only a very specific range of impact energies, either by changing the density or geometry (thickness) of the foam, but foams are not able to adapt their response to a wide range of impact energies. This can lead to a mismatch of the foam's functional capability to the impact energy, making the foam either “too soft” or “too hard” for the impact. A foam that is too soft (not dense enough) for an impact will compress too quickly or “bottom out” and transmit too much force to the impacted body. A foam that is too hard (too dense) for an impact will not compress enough and will decelerate the impacted body too quickly.
When foam becomes fully compressed under impact, it acts as a rigid body and loses its ability to absorb energy. The impact energy remaining after the foam is fully compressed is transmitted directly through the foam to the impacted body. A foam that is too soft for a given impact will compress too quickly, which allows large forces to be delivered to the impacted body and effectively decreases the functional distance and time over which the impact occurs. A Force/Time curve for a foam that is too soft for a given impact is shown in FIG. 2 of the accompanying drawing.
In the initial phase of impact, the foam does not slow the object enough, and this is represented by an early, only gradually increasing line segment on the Force/Time curve of FIG. 2, from 0 to 0.075 seconds. Next, during time period from 0.075 to 0.0125 seconds, the foam quickly compresses and packs down, at which point deceleration occurs in a short distance and time, shown as the spike in the curve of FIG. 2. This curve demonstrates that the majority of the deceleration occurs in a brief period of time and distance, thus delivering a high peak force, which is the most damaging to the impacted body. In addition, the potential for localized compression of the soft foam decreases the area over which the force may be transmitted, therefore potentially increasing the pressure and damage of the impact. Due to potentially catastrophic consequences of bottoming out within a small area, soft foams cannot be used in situations that may involve moderate or high energy impacts.
Conversely, a foam can also be too hard (too dense) for a given impact. If the foam is too hard, it will present too much resistance in the early phase of the impact, and will not compress enough (will not “ride-down” enough) to prolong the distance or time of impact. It thus halts the object suddenly, represented as the sharp continuous rise to a high peak force in the Force/Time curve shown in FIG. 3 of the drawing. This is most evident with respect to the curve labeled “Trial 1” in FIG. 3.
These dense foams function primarily to spread impact area and reduce pressure on the area, but can still lead to high forces. Another problem with dense foams is the potential for high “rebound,” in which the foam temporarily stores impact energy in compression, then re-delivers it upon rebound. Thus, dense foams are useful for reducing pressure of impacts, but their ability to significantly reduce peak force is limited.
Even when foams happen to be matched to the impact (which may occur by chance, or by specific engineering of foams to meet certain very specific energy level standards), they still have inherent limitations. One major limitation is the inability of the foam to “ride-down” enough to prolong the distance and time of the impact. Most foams will ride-down to a maximum of 60-70% of their original height, which limits the distance and time over which the impact occurs, and leads to higher peak forces. Given the limited ability to customize foams, for a given material operating at a given energy level, this presents only one option to further reduce peak forces. Specifically, the only way to further reduce peak forces is to lower the density of the foam and increase its height or thickness. This modification can serve to lower the peak forces, but due to the inherent properties of foam, which cause it to become progressively denser under compression, the curve is still hump- or bell-shaped, limiting the foam's ability to lower peak force. Further, an increased thickness of foam may be cosmetically or practically unacceptable for certain applications, and may also increase the bulk and weight of the energy management system to unacceptable levels.
Given the properties of foam, once it is manufactured, it will have a certain energy level at which it performs “optimally,” but this performance still leaves great room for improvement, and outside of its optimal range the foam's function will be even worse, either being potentially too hard or too soft for a given impact. Thus, foam lacks an ability to adapt to the potential for impacts of different energy levels. This leads to the use of foams designed simply to perform best at a certain standard, or designed to prevent only the most critical forms of damage, but leaving other forms of damage poorly addressed. FIG. 4 of the drawing includes two Force/Time curves for a given foam generated in response to two different impact energies. As is apparent from FIG. 4, the foam's performance declines with increased impact energy.
The second major limitation of foam is that all foams will show decline in function after repeated impacts. Some common foams, such as expanded polystyrene (EPS), are designed for only a single impact. Other foams, even though designed to be “multi-impact,” will also decline in function after repeated impacts. This lack of durability can present practical as well as safety limitations with the use of foams. FIG. 5 of the drawing includes a series of Force/Time curves for successive impacts to a “multi-impact” foam illustrating the decline in the foam's performance with repeated impacts.
In summary, the problems associated with foam as an impact-absorbing material include:                (a) limited adaptability;        (b) non-optimal impact energy management;        (c) tradeoff between energy absorbing ability and amount of material used; and        (d) poor durability.        While we have specifically focused on the limitations of foams, those skilled in the art will appreciate that other mechanisms of energy management may be employed, and that they may also be subject to the same or similar functional limitations as foam.        
There is thus a need in the art of impact energy management for a novel system capable of addressing the limitations of foams and other conventional energy management systems.